1. Field of the Invention
The present invention relates generally to the field of compliance devices and more particularly to overload protection devices for robots and other automatic machines.
2. Description of Related Art
Robots have been used for many years in industrial assembly line applications to perform repetitive tasks very precisely without the need for human operation, interaction, or supervision. For instance, robots are commonly used in the automotive industry to perform a number of tasks such as material handling and spot-welding of automobile bodies.
The robot often has a tool, either permanently or removably attached to the end of its arm, which enables it to perform its assigned function. Occasionally, the tool may encounter unexpected obstacles in the production line environment. If the tool impacts the obstacle with sufficient force, or if the robot continues to move once the tool has come into contact with the obstacle, the tool or the robot arm itself may become damaged. In order to avoid this undesirable result, devices are connected between the robot arm and the tool to give the tool a limited amount of freedom of movement (referred to as "compliance") when it encounters an obstacle. These devices, which may be referred to as "overload protection devices", often contain springs, air pistons, or other means for providing resilient compliance.
When the overload protection device complies as a result of the tool encountering an obstacle, this is referred to as an "overload condition." Most overload protection devices are provided with a plurality of sensors which detect an overload condition and let the robot know that it should stop moving until the overload condition is removed.
Because an obstacle can exert different forces on the tool depending upon where the two come into contact, it is desirable for an overload protection device to provide freedom of movement in as many directions as possible (e.g., freedom in the x, y, and z directions, as well as rotational freedom). The more degrees of freedom, the less likely the tool or the robot will be damaged.
Because the environments in which robots are used are sometimes hostile and unsafe for humans, it is also desirable for the overload protection device to be capable of automatically returning to its original position ("resetting") when the overload condition no longer exists, without the need for human intervention. Unfortunately, the more directions in which an overload protection device is capable of complying, the less likely it is to be capable of automatically resetting itself. This is particularly true of overload protection devices capable of rotational (twisting) compliance, since the device must be rotated back to the proper position.
Therefore, it would be desirable to provide an overload protection device capable of resetting itself regardless of whether the overload condition was due to linear or rotational movement.
Greater freedom of movement also normally means that more sensors are required to detect an overload condition, since displacement of the overload protection device can occur in a variety of directions. Each additional sensor, however, adds additional expense to the overload protection device. Therefore, it would also be desirable to provide an overload protection device that exhibits compliance in a plurality of directions, but requires only one sensor to detect an overload condition.
It would also be desirable to provide an overload protection device whose resilient compliance is softer during an overload condition than during non-overload conditions. This would allow the tool to move freely once it has broken free, thereby reducing the likelihood that damage will occur to the tool or to the robot.